At (or by) four he should be on a restricted 50cc. For youth you really need to go Suzuki. They are the most reliable in the youth atv market, and they hold their value well.
Do you ride Jake? If you intend to get him into the sport, and you are new to it, you should invest this spring in an ATV for yourself and take some safety courses. I mean you can jump right on one with a helmet and do just fine, but there are key fundamentals that teaching young riders is great because those fundamentals stick. For example, getting your body shifted into a turn and getting your weight over the gas tank.
Wish you were closer, I'd love to get out in the woods. I was going to go pick up my bike today from my mom's and hit the trails, but I got side tracked.